Introduction
Chapter 5 Corrugated Fiber Bragg Grating Structure: Strain
5.2 Experimental
Chapter 5: Corrugated Fiber Bragg Grating Structure: Strain Sensitivity Enhancement 99 The main aim of the research reported in this chapter is to design a highly reliable optical fiber strain sensor employing a unique FBG sensing structure, which would not only be capable of enhancing strain-sensitivity manifold, but would also facilitate single FBG to act as multiple independent sensors. To achieve this objective, a specific corrugated structure is carefully carved out over an otherwise uniform FBG using controlled etching process. This structure is exposed to the applied strain perturbations. Employed design strategy facilitates a single FBG to split into multiple independent FBG sensing elements. Most striking feature of this study is a manifold strain-sensitivity enhancement. Strain sensitivity of a particular section of the corrugated structured fiber in the FBG region, which acts as an independent sensor, is observed to be over 10.5 times higher than the sensitivity of an unetched (uniform) FBG region. This is a remarkable enhancement of strain sensitivity and is achieved for the first time to the best of author’s knowledge. Further, a maximum discrepancy of ±0.019 nm, strain resolution better than 0.0057 and a strain measurement accuracy of ~ ±2.15 are observed for this sensing structure. Experimental results also depict a highly reliable and reversible response characteristic for the proposed sensor.
Chapter 5: Corrugated Fiber Bragg Grating Structure: Strain Sensitivity Enhancement 100 more geometrical configurations reported in the literature and depicted in Fig. 5.1 (b) and (c), are also structured. To develop these strain sensors, four Bragg gratings written in a
Figure 5.1: FBG sensing configurations: (a) corrugated etched (b) fully etched & (c) half etched FBG along with schematics of applied strain perturbations, (d) FE-SEM image of the section of fiber, which was etched for 45 minutes.
(d) (b)
10mm
F
Fixed point
(c)
5mm 5mm
F
Fixed point (a)
3mm 3mm
4mm F
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Chapter 5: Corrugated Fiber Bragg Grating Structure: Strain Sensitivity Enhancement 101 germanium doped silica single mode fibers (125 µm diameter, E 7.21010 N/m2, A = 1.2310-8 m2, pe 0.22) using phase mask technique were used. All FBGs were 10 mm long and had a peak wavelength of 1545 nm. As depicted in Fig. 5.1, geometrical configuration for the proposed sensor along with the other two configurations on the part of fiber carrying FBG was structured by the sequential reduction of cladding diameter. Cladding reduction was achieved using standard chemical etching procedure that employed 45% HF solution. Special etching chambers of perspex sheets were designed in order to realize the three sensing geometries. Portion of the fiber carrying FBG was inserted inside a chamber suitable to realize a particular structure of proposed sensing configurations. Inserted fiber was exposed to HF solution with a careful protection at locations employing suitable epoxy resin. While etching, diameter of the fiber was monitored with an observed etching rate of ~1.6m/min. Fiber etching, carried out at room temperature (20 oC), was stopped after about 45 minutes. Afterwards, fiber diameter in the etched portion was examined by a field emission scanning electron microscope (FE-SEM). A uniform fiber diameter with a reduced value of ~42 µm was observed for the etched portion of the fiber in all the three sensing configurations. That is, fiber was etched to a depth of ~83µm in the cladding region, thus making the cladding in the etched part thinner by roughly 66%. As depicted in Fig.5.1 (a), fiber was etched in such a way that a 4 mm length of the fiber in the central part of FBG remained unetched; whereas the 3 mm length of the fiber on both the ends of this 4 mm central region of FBG existed in the etched portion for the proposed sensing configuration. Bragg wavelength of this structure was observed to be 1544.09 nm. For the two other configurations structured for comparative analysis, entire grating length of 10 mm existed in the etched portion of the fiber in one configuration (Fig.5.1 (b)) with a Bragg wavelength of 1543.93 nm; whereas fiber was etched in a way that half of the grating length (5 mm) existed in
Chapter 5: Corrugated Fiber Bragg Grating Structure: Strain Sensitivity Enhancement 102 the etched portion of the fiber and the other half remained unetched in the other configuration (Fig.5.1 (c)) with a Bragg wavelength of 1544.01 nm. For each of the configurations, diameter of the fiber in unetched part was 125 µm whereas the fiber diameter in etched part was ~42 µm.
5.2.2 Sensor characterization
I
n order to analyze the strain response of proposed sensors (FBG carrying core with differently etched cladding structures), a strain applying setup was developed. Figure 5.2 shows the schematics of experimental set-up.Figure 5.2: Experimental set-up for strain sensor.
Out of two vertical blocks used in this set-up, block ‘1’ was rigidly fixed to an optical table, whereas block ‘2’ was rigidly fixed over a one-dimensional translation stage. This translation stage was fixed to the optical table. Heights of the blocks were properly chosen in order to keep the top of both the blocks in the same horizontal plane. Distance between the two blocks in this arrangement was 20 cm. Translational stage was fixed in a way to have one degree of freedom - moving towards or away from the block ‘1’. The three differently etched FBG structures (Fig.
Fiber glued at these points
FBG Interrogator Movable
Block (“2”)
Stationary Block (“1”) Etched FBG
(Strain Sensor) FBG
(Temperature Sensor)
Optical Table
Chapter 5: Corrugated Fiber Bragg Grating Structure: Strain Sensitivity Enhancement 103 having two sensors each – etched FBG structure as sensor 1 and unetched FBG as sensor 2. The unetched FBG was identical (central wavelength 1558.62nm) in all the three sets and was used to monitor temperature variations during the experiment. In order to investigate the response characteristics of the proposed sensor, fiber carrying corrugated etched FBG structure (Fig.
5.1(a)) and unetched FBG was fixed onto the blocks in such a way that the corrugated etched FBG was exactly in the middle of blocks ‘1’ and ‘2’. The part of the fiber carrying unetched FBG was in the right side of block ‘1’. Fiber end in this side was connected to FBG interrogator (Micron Optics, SM130-700, resolution 0.05pm). Fiber carrying corrugated etched FBG structure was replaced by the fiber carrying etched FBG structures of Fig. 1(b) and 1(c) respectively for the experiments carried out for comparative analysis. Before starting the experiment, portion of the fiber in between blocks ‘1’ and ‘2’ was ensured to be straight and then was suitably pre-strained in order to avoid any infinitesimal amount of slacking. At this pre- strained level, the applied strain was calibrated as zero. After words, block ‘2’ was slowly moved towards left in order to apply forward (increasing) strain perturbations to the etched FBG structures in all the three experiments. After reaching to the maximum sustainable strain perturbation, block ‘2’ was then slowly brought back to the calibrated zero strain position (reverse strain perturbations). Corresponding Bragg wavelength shifts of these strain sensing structures for forward as well as reverse strain perturbations along with the wavelength response of the multiplexed unetched FBG were then recorded through the interrogator. As no strain perturbation was applied to the unetched FBG, its wavelength response simply manifested the temperature variations. Wavelength response of unetched FBG was subtracted from the wavelength response of etched FBG structure in order to compensate the effect of temperature
Chapter 5: Corrugated Fiber Bragg Grating Structure: Strain Sensitivity Enhancement 104 variations during a particular experiment. It is this temperature insensitive response for the strain sensor (etched FBG structures) which was then analyzed.